Chronic viral infections are a significant cause of morbidity and mortality. CD8 T cell exhaustion is a common occurrence during many chronic infections and this state of dysfunction prevents optimal pathogen control. Reversing o avoiding T cell exhaustion is central to therapeutic and preventative strategies to tret chronic viral infections such as HIV, HCV, HBV and many others. The underlying molecular mechanisms of T cell exhaustion, however, remain poorly understood. We have recently defined progenitor and terminal subsets of exhausted CD8 T cells that are involved in maintaining immunity during chronic viral infections in mice and humans. The overall goal of this proposal is to define the lineage dynamics and transcriptional regulation of these subsets and determine how their control regulates exhaustion and reversibility of function during chronic infections. Several major unanswered questions will be addressed. First, we will define how the severity of chronic infection and the immunoregulatory molecule PD-1 impacts the lineage dynamics, maintenance and fate flexibility of exhausted CD8 T cells subsets. Second, we have identified T-bet and Eomes as the two key transcription factors controlling the progenitor and terminal subsets of exhausted CD8 T cells, but precisely how these transcription factors function is unclear. We will use temporal conditional deletion or overexpression to test when these two transcription factors become critically important to exhausted CD8 T cells and to define when T-bet and Eomes develop context specific function in acute versus chronic infection. Third, we will use conditional deletion and overexpression combined with systems-biology approaches to define the mechanisms of temporally distinct transcriptional coordination by T-bet and Eomes during T cell exhaustion. Thus, the overall hypothesis of this proposal is that the balance between progenitor and terminal subsets of exhausted CD8 T cells controls irreversible commitment to CD8 T cell exhaustion through temporally acquired unique functions of T-bet and Eomes. We will test this hypothesis in the following specific aims: AIM 1: To define the signals that regulate progenitor versus terminal populations of exhausted CD8 T cells during chronic viral infection. AIM 2: To test whether T-bet and Eomes have temporally distinct functions during acute versus chronic infection. AIM 3: To define the transcriptional mechanisms underlying the role of T-bet and Eomes in CD8 T cells exhaustion versus memory.